Several concerns exist for bringing a foldable/rollable iPhone to market:
1. Foldable/rollable smartphones are a niche product category today. Who will buy them?
The especially large Chinese market will, due to their preference for large-screen phones.
According to BusinessInsider, "For many Chinese, the phone is also the first, the most important, or even the only computing device they own. "People just use them for more, and therefore appreciate the bigger screen," said Frank Gillett, an analyst at technology research firm Forrester."
2. Do foldable/rollable smartphones improve the user experience?
Foldable/rollable smartphones do improve the user experience for those who want the biggest screen they can fit in their pocket.
3. What does it take to stay competitive in the smartphone market today?
Apple needs to keep their iPhone lineup as interesting as their competition's. Only selling 6.1-6.7 in candybar smartphones is unimpressive when Samsung sells those and a variety of interesting foldable designs as well. Samsung's lineup is more interesting right now, so Apple needs to compete with that.
4. Don't products need to be "market-ready"?
It's true a product should have a certain polish to it before being sold. But smartphone companies introduce new models every year, allowing them to make yearly improvements. Apple will be able to make yearly improvements on their foldable iPhones like they've been doing for their iPhone lineup every year since the very first iPhone.